It's time to quit relying on Lincoln's head.
That's the contention of a mechanical engineer, who recommends killing off the widely accepted 2/32-inch tread depth rule. For as long as people can remember, motorists have decided it's time to replace tires by inserting a penny upside-down in the tread: If the top of Lincoln's head or any of the words ``In God We Trust'' show, its time to get some new rubber.
But like the bias-ply tire, the Lincoln measuring method is an anachronism that should be changed, according to William Blythe. He challenged the long-held theory of what minimum tread level depth is safe in a presentation on Sept 12 at the ITEC 2006 tire conference and exhibition in Akron.
Blythe, who has taught applied mechanics at San Jose State University for 42 years, based his recommendations on laboratory tests. He said the studies show wet traction declines significantly as tread depth falls below 4/32 inch.
It's an issue with some other interesting facts. Such as, according to Blythe, there doesn't seem to have been any rhyme or reason for the 2/32-inch rule-of-thumb. That's just what everyone went with.
You might ask, ``What is the federal standard for minimum tread depth?'' Well, there isn't any. The closest thing is a regulation that tires sold in the U.S. must have wear indicators that show when a tire has worn to 2/32nds inch. Which a consumer may, or more likely, may not, pay any attention to.
Canada has a 2/32-inch federal standard, which the police can enforce. And in Europe, a movement is afoot to study and review the situation.
Blythe's peers who heard his presentation generally believed his study has merit and called for further, real-world testing.
A few years back, before the era of tire recalls, before tire safety grabbed headlines, before the creation of the TREAD Act, would much consideration be given to the minimum tread depth of a tire? The answer: No, and the proof is the still-used 2/32-inch rule.
It's a different world today when it comes to tire safety. That's not a bad thing.